Creates a provisional data source that will transition to a committed data source once any data has been received.
Use webFrameStopLoading to stop the load.
This function is typically invoked on the main frame.

You use the webFrameLoadRequest method to initiate a request that creates a provisional data source.
The provisional data source will transition to a committed data source once any data has been received.
Use webFrameGetDataSource to get the committed data source.